With increasing intensity of circuit integration in semiconductor devices in recent years, circuit lines have become finer and interline spacing has also been drastically reduced. With this trend for finer resolution in circuit fabrication, it is now necessary to provide a precision flat substrate surface because of the extreme shallow depth of focus required in optical photolithography using stepper reproduction of circuit layout. One method of obtaining a flat surface is mechano-chemical polishing carried out by pressing wafers held on a carrier against a polishing cloth mounted on a rotating turntable while dripping a solution containing abrasive powder at the interface of the wafer and the polishing cloth.
FIG. 11 shows a polishing apparatus disclosed in a Japanese Patent Laid-Open Publication, H9-117857. The facility is comprised by a pair of polishing units 101a, 101b disposed symmetrically at one end of a rectangular-shaped floor, and a loading/unloading unit including wafer cassettes 102a, 102b disposed on the opposite end of the floor for storing wafers. Transport rails 103 are disposed along a line joining the polishing units 101a, 101b and the loading/unloading unit, and alongside the rails 103, there are wafer inverters 105, 106 surrounded by respective cleaning units 107a, 107b and 108a, 108b. 
Such a polishing apparatus, comprised by a pair of parallel processing lines arranged on both sides of the rails, is able to handle workpieces polished through a single step process in each line of the facility to improve its productivity. For those workpieces requiring a double step polishing, such as compound semiconductor materials requiring polishing steps using different solutions, after completing a first polishing step through one polishing line 101a, the workpieces are cleaned next, and then transferred over to the second line 101b to carry out a second polishing step. Thus, such a polishing apparatus is able to carry out a series-operation for workpieces processed in double-step polishing, and a parallel-operation for workpieces processed in single-step polishing.
Transport of workpieces in the parallel polishing process is carried out as follows. After completing a polishing operation of the polishing units 101a, 101b, the top ring (workpiece carrier) 110 rotates and moves over to the workpiece pusher (transfer device) 112 to transfer the polished workpiece. A second robot 104b transports the workpiece over to the cleaning units 107a or 107b, and receives an unpolished workpiece from the inverter 105, 106, and transfers it to the workpiece pusher 112. The top ring 110 receives the unpolished workpieces and moves back to the turntable 109 to begin polishing. A dresser 111 is provided to carry out reconditioning of a polishing cloth.
A polishing unit, such as the one shown in FIG. 12, is comprised by a turntable 109 having a polishing cloth 115 bonded to its top surface, and a top ring 113 for holding and pressing a wafer W onto the turntable 109. Polishing action is produced by rotating and pressing the wafer W by the top ring 113 against the rotating turntable 109 while a polishing solution Q is supplied in the interface between the wafer W and the polishing cloth 115. The polishing solution Q is held between the surface to be polished (bottom surface) of the wafer W and the polishing cloth 115 while the wafer is being polished.
In such a polishing unit, the turntable 109 and the top ring 113 are rotated at their own independent speeds, and the top ring 113 is positioned, as shown in FIG. 12, so that the inner edge of the wafer W will be off from the center of the turntable 109 at a distance “a”, and the outer edge of the wafer W will be at a distance “b” from the periphery of the turntable 109, respectively. The wafer W is polished in this condition at high rotational speeds so that the surface of the wafer will be polished uniformly and quickly. Therefore, the diameter “D” of the turntable 109 is chosen to be more than double the radius “d” of the wafer W according to the following expression:D=2(d+a+b).
Polished wafers W are stored in the wafer cassette 102a, 102b after having gone through one or more cleaning and drying steps. Cleaning methods for wafers include scrubbing with brush made of nylon or mohair, and sponges including polyvinyl alcohol (PVA).
One of the problems in the existing polishing apparatus is its productivity. To increase the throughput from such a facility, the efficiency-determining processes involving polishing at the turntable 109 must be raised. However, in the existing technology, one robot 104b is required to carry out a multiple duty of removing polished wafers and supplying unpolished wafers to and from two workpiece pushers 112. This is time-consuming, resulting in idle time for the turntable 109.
Therefore, there is a need to provide, as a first objective, a polishing apparatus having two parallel processing lines that carries out efficient parallel processing by minimizing the idle time for the turntable and maximizing the through-put.
Furthermore, in the existing polishing apparatus, a high relative speed between the turntable 109 and the top ring 113 is used to achieve effective polishing as well as high flatness of the wafer surface, but this high relative speed may also cause micro-scratch marks on the wafers due to abrasive particles contained in the polishing solution.
To prevent fine scratches, it is possible to consider utilizing two sets of turntables 109, and carry out polishing in two stages, by changing polishing parameters such as the material and abrasive characteristics of the polishing cloth 115, rotation speed of the turntable 109, and polishing solution. However, as mentioned above, the large size of the turntable 109 occupying a large installation space and requiring high capital cost are disadvantages of such an approach, and this type of problem is expected to become more serious in the future, as larger diameter wafers become more common.
On the other hand, it is also possible to consider using one turntable by switching polishing solutions or by reducing the rotational speed to resolve existing problems, but such approaches are not expected to lead to improved productivity, because mixing of solutions may lead to poor performance and polishing time would be lengthened.
Another problem is related to cleaning of the wafers. When the wafers are scrubbed after polishing with abrasive particles, it is difficult to remove particles of sub-micron sizes, and if the adhesion force between the wafer and particles is strong, such cleaning method is sometimes ineffective for removing such particles.
Therefore, there is a need to provide, as a second objective, a compact polishing apparatus that can provide excellent flatness and efficient cleaning.